Schizophrenia is a common and devastating mental disorder which is currently an unmet medical need. It is characterized by so-called positive (hallucinations, delusions) and negative (blunted affect, poverty of speech, social & emotional withdrawal) symptoms, as well as cognitive deficits (working memory impairment). About 1% of the world population is affected, men and women equally, with typical onset between ages 15 and 25. Antagonists of the neurotransmitter dopamine are known to block psychosis. The present invention provides compounds of formula I (wherein each R is independently C1-8 alkyl), a highly selective D3 receptor antagonist, for the treatment of Schizophrenia and other CNS diseases. 
Racemic forms of formula I and their preparations have been disclosed in PCT publication WO 97/45403. The present invention has discovered that the (2S)-enantiomer of formula I is the form that possesses the superior desirable bioactivity. The present invention also provides a process for the synthesis, in a large scale, of said (2S)-enantiomer in a highly enantiomerically enriched form, which solved an extremely challenging problem of a long period of time.